Electron discharge tubes

ABSTRACT

An electron discharge tube, for example a travelling wave tube, which employs a periodic magnetic field, is provided with at least one pair of adjacent magnets with larger spacing in the axial direction than is employed between the magnets of other adjacent pairs. The perturbation of the field due to the larger spacing is reduced by providing only one pole piece interposed between the two magnets on each side of the larger spacing. The remaining pairs of magnets may have normally spaced pole pieces. The larger spacing may be occupied by a waveguide connection.

United States Patent Kennedy et al.

[54] ELECTRON DISCHARGE TUBES [72] Inventors: Matthew Campbell Kennedy; Peter Tourle 211 Appl. No.: 71,135

[52] US. Cl "313/153, 31513.5, 315/535,

[ Feb. 22, 1972 3,259,787 7/1966 Fentonetal ..3l5/3.5

3,317,780 5/1967 Ayers ....3l5/3.5 3,551,729 12/1970 Bradford ..315/3.5

[ ABSTRACT An electron discharge tube, for example a travelling wave tube, which employs a periodic magnetic field, is provided 332/210 with at least one pair of adjacent magnets with larger spacing [51] Illt. Cl. .110117/00, H013 3/04 in the axial direction than is employed between the magnets of [58] Fleld of Search ..315/3.5, 5.34, 5.35, 39.3; other adjacent pairs The perturbation of the field due to the 335/210 313/153 larger spacing is reduced by providing only one pole piece interposed between the two magnets on each side of the larger [56] References cued spacing. The remaining pairs of magnets may have normally UNITED STATES PATENTS spaced pole pieces. The larger spacing may be occupied by a waveguide connection. 3,188,533 6/1965 Brctting et al. ..315/3.5 X 3,355,622 11/1967 Bradford ..315/3.5 7 Claims, 4 Drawing Figures 7 2 2 2 2 2 2 7 7 7 7 a A 1 1 l l K Z W L 3 4 5 4 3 /l l I A l PATENTEDFEB22 I972 3, 644, 771

SHEET 1 OF 2 NNSSN FIG/f AXIAL MAGNET/C FIELD STRENGTH F/GZ.

INVENTORS )Waflw m 7%1 BY 612m mwm ATTORNEYS ELECTRON DISCHARGE TUBES This invention relates to electron discharge tubes and more specifically to electron discharge tubes of the kind employing so-called periodic magnetic field systems. Probably the best known example of tubes of this kind is provided by the travelling wave tube.

The electrons from the cathode of a normal known travelling wave tube are accelerated by an electrostatic field, focused into an electron beam and finally collected by a collector. For correct operation the electron beam must be constrained to lie within a closely defined region about the axis of the tube. Left to themselves the electrons would, of course,-

spread laterally by mutual repulsion and proper focusing of the beam would not be obtained. Normal practice is to prevent this by an axial magnetic focusing field produced by ring magnets surrounding the envelope of the tube. Such a field can be produced by a simple tubular magnetic system but this has the defect of producing large leakage of magnetic flux. For this reason focusing is commonly effected by a periodic magnetic system comprising a number of permanent magnets and pole pieces in alternating succession of polarity (i.e., .with neighboring magnets in opposite polarity arrangement) along the axis. A typical known periodic magnetic system of this nature is illustrated in FIG. 1 of the accompanying drawings. This shows a part of the magnetic system near the normally provided input or output waveguide l, permanent magnets being shown at 2 and their pole pieces at 3. T is part of a support tube which coaxially surrounds the envelope (not shown) of the tube.

It will be seen that with this arrangement the waveguide (it could be either the input guide or the output guide) is interposed between two of the permanent magnets which are, therefore, spaced farther apart along the axis of the tube than are other adjacent magnets. Such added spacing results in a hesitation in the periodicity of the magnetic field and this is detrimental to the obtaining of correct focusing. The effect on the beam depends largely on the current density of the beam, the mean velocity of the electrons thereof and on the relative dimensions of the waveguide and the magnets. In normal present day practice the waveguide dimension in the direction parallel to the axis of the tube is reduced as much as possible in order to keep hesitation or interruption of the magnetic field as little as possible. This is a practically satisfactory expedient in the case of tubes designed for relatively low frequencies for, with such tubes, the tube dimensions are relatively large and the perturbation effect due to the inserted waveguides sufficiently small not seriously to affect the proper operation of the tube. This, however, is not the casewith highfrequency tubese.g., tubes for operation at frequencies of the order of 6 gc./s. or higher and at power levels below watts-the period of reversal of the focusing magnetic field system is small and the input and output waveguides produce serious interruption adversely affecting operation of the tube. FIG. 2 of the accompanying drawings exemplifies the sort of perturbation obtained in such cases, there being, as will be seen, a marked region in the neighborhood of the inserted guide where the magnetic field is under reduced control and tends to diverge from the axis. (This region is shown in the part of FIG. 2 vertically beneath the showing of the guide in FIG. 1).

A known method of avoiding or reducing this defect consists in providing additional bar magnets suitably positioned near an inserted guide with their axes parallel to the tube axis and their polarities opposite to those of the nearest neighboring magnets, the additional magnets being dimensioned and arranged to compensate as closely as possible for the irregularity, due to the inserted guide, of the spacing of the magnets of the periodic system. This expedient has, however, the serious practical disadvantage of making the magnetic system of considerably larger dimension in the cross section.

The present invention seeks to overcome or substantially reduce perturbation of the field ofa periodic magnetic system in a tube of the kind referred to-especially a high-frequency travelling wave tube-without introducing the defects of the known expedient of using additional magnets.

According to this invention an electron discharge tube employing a periodic magnetic field system includes at least one pair of adjacent magnets with larger spacing in the axial direction than is employed between the magnets of other adjacent pairs wherein perturbation of the field due to said larger spacing is reduced or prevented by providing only one pole piece interposed between the two magnets which are one each side of said larger spacing, said pole piece being adjacent to one of said magnets so that the spacing is left between that pole piece and said other magnet.

Preferably thereare two pole pieces additional to said interposed pole piece and each is adjacent to one or other of the said two magnets and outwardly thereof in respect of the spacing thereof, said two pole pieces being of L section with the bases of the L's extending towards the said spacing.

Preferably, in practicing the invention, the aforesaid spacing is used to accommodate an input or output waveguide.

The invention is illustrated in and further described with reference to FIGS. 3 and 4 of the accompanying drawings, in which FIG. 3 is a diagrammatic section illustrating the magnet arrangement of one embodiment of this invention, and FIG. 4, which should be compared with FIG. 2, is provided to show the sort of improvement obtainable with the aid of the inventron.

Referring to FIG. 3, which is a view similar to that of FIG. 1 (showing a known arrangement), the periodic magnet system comprises alternately poled permanent ring magnets 2, separated from each other by pole pieces 3, 4 and 5. Only part of the periodic system is shown and (except where input and output waveguides are interposed) the pole pieces are of inverted T section, i.e., like the pole piece 3. An interposed waveguide is shown at lit could be either the input or the output waveguide of a travelling wave tubeand because of its interposition the magnets 2 on either side thereof have larger spacing longitudinally than is adopted generally over the periodic system. In a known arrangement as shown in FIG. 1 this would cause perturbation of the field as exemplified by FIG. 2. However, in the embodiment of FIG. 3 there is an asymmetric arrangement on each side of the interposed guide, the asymmetry being chosen to compensate for the perturbation which would otherwise occur. This asymmetry is achieved in the illustrated embodiment of FIG. 3 by providing a pole piece on one side only of the guide 1 between said guide and the magnet 2 on one side thereof but no pole piece on the other side of said guide between it and the nearest magnet 2 on that side. This one pole pieces is the pole piece 5. It is almost annular but has a small step," as shown, to locate the guide 1. Also the next pole pieces outwardly-the pole piece 4-are of L section with the bases of the Us towards the space where the waveguide is interposed. The actual dimensions to secure minimum perturbation can be selected in accordance with known principles but, in practice, it is probably easy to finalize a design by trial and error. By suitable design it is possible to achieve a quite close approximation to zero perturbation as illustrated graphically in FIG. 4.

In practice the L-sectioned pole piece 4 on the same side of the waveguide l as the pole piece 5 can be substituted by a pole piece of inverted T section (like the pole piece 3) with but little degradation of focusing.

Although the main application of the invention is to travelling wave tubes it is applicable to other tubes of the kind referred to-notably to klystrons.

We claim:

I. An electron discharge tube employing a periodic magnetic field system including at least one pair of axially magnetized adjacent magnets having oppositely sensed magnetic polarity and at least one further magnet on each side of said one pair to provide, respectively with the magnets of said one pair, second and third pairs of magnets adjacent said one pair, said one pair having larger spacing in the axial direction than is employed between the magnets of said second and third pairs wherein perturbation of the field due to said larger spacing is minimized by providing only one pole piece interposed between the two magnets of said one pair, said pole piece being adjacent to one of said magnets of said one pair so that a space is left between said pole piece and the other magnet of said one pair and wherein a region of minimum magnetic field strength is produced within said larger spacing.

2. A tube as claimed in claim 1 wherein there are two further pole pieces each adjacent to one of the said two magnets of said one pair and outwardly thereof in respect of the spacing thereof, said two further pole pieces being of L section with the bases of the Us extending towards the said larger spacing.

3. A tube as claimed in claim 1 wherein the aforesaid larger spacing is used to accommodate an input or output waveguide.

4. In a system for focusing an electron beam within an electron discharge tube, the combination of:

a series of coaxially aligned spaced annular magnets to produce a periodic magnetic field for focusing purposes, said magnets each having axially spaced N and S poles and said magnets being arranged such that like poles of adjacent magnets are facing each other, an intermediate adjacent pair of said magnets being spaced apart a selected amount and the spacing between the remaining adjacent pairs of magnets in the series being equidistant and less than said selected spacing whereby the equidistant spacing establishes a selected periodicity to between said other magnet and that magnet adjacent thereto which, with said other magnet, constitutes a magnet pair next adjacent said intermediate pair, said second pole piece being of L-shaped cross section with the base of the L extending within said other magnet.

6. In a system as defined in claim 5 wherein said means further includes a third annular pole piece located and bridging between said one magnet and the next magnet adjacent thereto, said third pole piece being of L-shaped cross section with the base of the L extending within said one magnet.

7. In a system as defined in claim 6 wherein further annular pole pieces are disposed between the remaining magnets of said series, each of said further pole pieces being of T-shaped cross section with the bar of the T extending within the magnets on either side thereof. 

1. An electron discharge tube employing a periodic magnetic field system including at least one pair of axially magnetized adjacent magnets having oppositely sensed magnetic polarity and at least one further magnet on each side of said one pair to provide, respectively with the magnets of said one pair, second and third pairs of magnets adjacent said one pair, said one pair having larger spacing in the axial direction than is employed between the magnets of said second and third pairs wherein perturbation of the field due to said larger spacing is minimized by providing only one pole piece interposed between the two magnets of said one pair, said pole piece being adjacent to one of said magnets of said one pair so that a space is left between said pole piece and the other magnet of said one pair and wherein a region of minimum magnetic field strength is produced within said larger spacing.
 2. A tube as claimed in claim 1 wherein there are two further pole pieces each adjacent to one of the said two magnets of said one pair and outwardly thereof in respect of the spacing thereof, said two further pole pieces being of L section with the bases of the L''s extending towards the said larger spacing.
 3. A tube as claimed in claim 1 wherein the aforesaid larger spacing is used to accommodate an input or output waveguide.
 4. In a system for focusing an electron beam within an electron discharge tube, the combination of: a series of coaxially aligned spaced annular magnets to produce a periodic magnetic field for focusing purposes, said magnets each having axially spaced N and S poles and said magnets being arranged such that like poles of adjacent magnets are facing each other, an intermediate adjacent pair of said magnets being spaced apart a selected amount and the spacing between the remaining adjacent pairs of magnets in the series being equidistant and less than said selected spacing whereby the equidistant spacing establishes a selected periodicity to the magnetic field; and means for substantially reducing perturbation of said magnetic field in the region of said intermediate adjacent pair of magnets so as substantially to retain said selected periodicity throughout said series, said means comprising a single annular pole piece disposed adjacent one magnet of said intermediate pair and being of an axial thickness to present a space between it and the other magnet of said intermediate pair.
 5. In a system as defined in claim 4 wherein said means also includes a second annular pole piece located and bridging between said other magnet and that magnet adjacent thereto which, with said other magnet, constitutes a magnet pair next adjacent sAid intermediate pair, said second pole piece being of L-shaped cross section with the base of the L extending within said other magnet.
 6. In a system as defined in claim 5 wherein said means further includes a third annular pole piece located and bridging between said one magnet and the next magnet adjacent thereto, said third pole piece being of L-shaped cross section with the base of the L extending within said one magnet.
 7. In a system as defined in claim 6 wherein further annular pole pieces are disposed between the remaining magnets of said series, each of said further pole pieces being of T-shaped cross section with the bar of the T extending within the magnets on either side thereof. 